This invention generally relates to energy beam induced layer disordering (EBILD) and more particularly to the use of a direct write energy beam to selectively melt regions in a semiconductor heterostructure to produce a pattern of substantially homogeneous alloy useful for device fabrication.
Compound semiconductor samples intended for device fabrication are grown in the form of a single crystal comprised of two or more heterogeneous layers of different alloy composition, thickness and impurity concentration and/or type. By varying these crystal parameters, the electrical and optical properties of the crystal layers determine the device structure in the direction perpendicular to the plane of the as-grown layers. Further, these modifications must be selectively applied parallel to the plane of the as-grown layers with an appropriate pattern of micron resolution. Typically, conventional photolithographic techniques have been extensively employed to effect such lateral modifications. As an example, photolithographically patterning zinc or silicon or GaA1As is followed by surface initiated inpurity induced disordering (IID) via high temperature diffusion of an impurity that causes layer disordering. See W.D. Laidig et al, "Disorder of an A1As-GaAs Superlattice by Impurity Diffusion", Applied Physics Letters, Vol. 38(10), pp. 776-778, May 15, 1981 and U.S. Pat. No. 4,378,255 (zinc diffusion), and J. J. Coleman et al, "Disorder of an A1As-GaAs Superlattice By Silicon Implantation", Applied Physics Letters, Vol. 40(10), pp. 904-906, May 15, 1982 and U.S. Pat. No. 4,511,408 (silicon ion implantation or silicon diffusion). IID is a process wherein an impurity, e.g. Si, is thermally diffused through two or more heterostructure layers of a sample, such as GaA1As multilayers, disordering the isoelectronic constituents of the layers, for example Ga-A1 interdiffusion of the AaA1As layers, so that a new homogeneous alloy is formed in regions of the layers subjected to the diffusion treatment. The details of the disordering mechanism are not fully understood at this time, e.g. why the Si impurity causes an interdiffusion of elements to rapidly occur. Patterning is accomplished by the use of masking techniques so that diffusion is carried out in selected regions exposed to the impurity during the diffusion process. The disordering impurity may also be ion implanted through a mask wherein the ions implanted are a species of the ion beam. This process is followed by annealing to bring about disordering of the heterostructure layers in the regions of implant as well as anneal out the damage that occurs which is not always possible. In either case, the disordering is carried out under temperatures that are below that required for ordinary significant A1-Ga interdiffusion without the diffused or implanted impurity.
Thus, IID provides a patternable process for locally mixing alloy semiconductor layers without compromising crystal quality. The migration of the implanted or diffused impurity via thermal anneal causes traverse (traverse to the parallel extent of the as-grown crystal layers) modification to the bandgap, wavelength or refractive index of the heterostructure regions of a sample subjected to the treatment. IID has been used successfully in fabricating low threshold buried heterostructure (BH) lasers and is capable of fabricating other optoelectronic devices such as optical waveguides. In this connection, see the article of Robert L. Thornton et al, "Optoelectronic Device Structures Fabricated by Impurity Induced Disordering", Journal of Crystal Growth, Vol. 77, pp. 612-628, 1986. We refer to this IID process as "surface initiated IID" since, in the practice of the process, the impurity is diffused into the surface of the sample.
A problem in the application of surface initiated IID, however, is that the depth or lateral extension or width of the disordered region is not easily predictable or determinable relative to the length of time for diffusion or anneal. Further, the employment of surface initiated IID as taught by these references requires the use of masking and the generation of masks, which is a time consuming and expensive intermediate step and is dimensionally limited by the mask generation capability or facilities employed.
A direct write energy beam technique would be more efficient and useful because it has the advantage that complex patterns could be computer generated and the beam selectively applied to regions of the semiconductor structure during epitaxial growth or device preparation followed by device testing without the need of an intermediate step of mask generation. Also, with a direct write system, a commercially available low power cw beam provides a sifficiently high energy density for melting the crystal structure without significant accompanying damage to the crystal. Also, there is no problem of improper pattern registration since the beam is indexed relative to the sample via highly accurate computer control. Further, as described below, a direct write technique can be directly applied to effect immediate change to the semiconductor crystal to a practical dapth of about 1.5 .mu.m.
This is not to say that that research and development efforts in the art have not been accomplished relative to beam assisted direct writing techniques. For example, U.S. Pat. No. 4,159,414 discloses a method to provide laser induced changes in a previously deposited compound by reducing the compound wherein selected regions of the deposited compound are reduced to an elemental state, e.g. a metal. Also, in the article of L. D. Laude, "Laser Induced Synthesis of Compound Semiconductors", Material Research Symposium Proceedings, Vol. 23, pp. 611-620 (1984), the constituents of compounds, such as, A1Sb, A1As, CdTe, CdSe, ZnTe, ZnSe and GeSe.sub.2, are, first, independently evaporated onto a glass substrate as separate thin amorphous films and then subjected to a laser beam to transform them, via what its believed to be a solid phase process, i.e. no melting is occurring, into a crystalline compound. The process involves the local formation of an alloy per se and not the local modification of an alloy in a system.
Another area of beam assisted direct write technique has been the use of laser source energy in photolysis and pyrolysis to effect deposition of a material form its gaseous phase onto a substrate surface. In the case of photolysis, a gaseous compound of the material to be deposited is introduced into a chamber above the surface of a substrate and a laser source of energy is used to decompose the gaseous compound. The compound absorbs a portion of the incident laser energy at the selected wavelength causing photo decomposition of the compound close to the surface of the substrate and releasing a constituent of the compound for deposition on the substrate surface. As examples, see Pat. Nos. 4,606,932 and 4,340,617, and the work of D. J. Ehrlich et al at Lincoln Laboratory at MIT, e.g. D.J. Ehrlich et al, "Direct Writing of Regions of High Doping On Semiconductors By UV-Laser Photodeposition", Applied Physics Letters, Vol. 36(11), pp. 916-918, June 1, 1980 and D. J. Ehrlich et al, "Summary Abstract: Photodeposition of Metal Films With Ultraviolet Laser Light", Journal of Vacuum Science Technology, Vol. 20(3), pp. 738-739, March, 1982. In the case of pyrolysis, the gaseous compound decomposition is assisted by heating the substrate surface. As examples, see Irving P. Herman et al, Materials Research Society Symposia Proceedings, Re: Laser Diagnostics and Photochemical Processing For Semiconductor Devices, Volume 17, pp. 9-18(1983). A combination of photolysis and pyrolysis is taught in U.S. Pat. No. 4,505,949.
Another class of beam assisted direct write techniques is laser annealing to remelt an amorphous or polycrystalline material, such as amorphous silicon or amorphous GaAs, and convert such material into a single crystal material. See, for example U.S. Pat. Nos. 4,330,363 and 4,388,145.
A further class of beam assisted direct write techniques is laser annealing for the restoration of crystal structure which has been damaged by previous impurity implantation as well as for the electrical activation of the implanted impurity. See, for example, G. A. Kachurin et al, "Annealing of Implanted Layers by a Scanning Laser Beam", Soviet Physics Semiconductor, Vol. 10(10), pp. 1128-1130, October, 1976, and E. I Shtyrkov et al, "Local Laser Annealing of Implantation Doped Semiconductor Layers", Soviet Physics Semiconductor, Vol. 9(10), pp. 1309-1310, 1976 and U.S. Pat. No. 4,151,008.
Beam assisted direct write techniques have also been applied to melting doped semiconductor surface layers via laser pulsed radiation to redistribute the impurity in the layer as per U.S. Pat. Nos. 4,181,538 and 4,542,580, and G. A. Kachurin et al, "Diffusion of Impurities as a Result of Laser Annealing of Implanted Layers", Soviet Physics Semiconductor, Vol. 11(3), pp. 350-352, March, 1977. Also, a focused laser beam has been employed to diffuse an impurity from region of semiconductor structure into another region without melting as exemplified in U.S. Pat. No. 4,318,752.
Of course other types of energy beams have been used for beam assisted direct write techniques. Electron beam techniques are commonly used in lithography for mask generation and photoresist exposure. Ion beam techniques have been developed for maching tooling or etching or as an implantation tool as per U.S. Pat. No. 4,334,139 and K. Ishida et al, "Fabrication of Index-Guided A1GaAs MQW Lasers by Selective Disordering Using Be Focused Ion Beam Implantation", Japanese Journal of Applied Physics, Vol. 25(9) pp. L783-L785, September, 1986.
Thus far the successful development of energy beam assisted direct write techniques for practical accomplishment of patterned thermal disordering or impurity induced disordering relative to semiconductor heterostructures has not been accomplished.